A significant proportion of patients presenting to the TMJ & Craniofacial Pain Clinic at the University of Minnesota complain of orofacial pain or headaches that can be precipitated, aggravated, or modified by palpation of localized areas over the masticatory muscles. The etiology of this phenomenon is unknown. Pathophysiological processes occuring within the masticatory muscles may be contributing to these symptoms by activating and/or sensitizing muscle nociceptors. Steen et al. ( 1992) showed that changes in pH activate cutaneous nociceptors. Bengtsson et al. (1986) reported biochemical changes, suggestive of hypoxia, in biopsy specimens taken from tender muscle. Therefore, localized areas of hypoxia may lead to decreased pH which may activate and/or sensitize muscle nociceptors leading to myalgia. My long-term goal is to gain an understanding of the anatomical pathways and physiological processes involved in muscle nociception and to apply this knowledge to elucidating the etiologic processes underlying these myalgias. I have not started a dissertation research project but I am participating in a research project in which the long-term goal is to develop methods for measuring pH in vivo. We use an active microelectrode, a KCI reference electrode with a buffer bridge, and a high impedance amplifier connected to a computer acquisition system to measure and record pH in rat hindlimb muscle. In previous experiments, we demonstrated that these microelectrodes can be used to reliably and validly measure the pH of physiological buffer solutions (Gunn et al. 1993). Currently, we are investigating the reliability and validity of our pH measurement system in muscle. Once these measurements are accomplished we will monitor muscle pH changes in response to various ischemic conditions while electrophysiologically recording the activity of nociceptive muscle afferent units . These experiments will test the hypothesis that decreased muscle pH can activate and sensitize muscle nociceptors.